United States patent published under U.S. Pat. No. 4,617,676 describes a digital communications system receiving on its input a speech signal that has a frequency spectrum featuring slopes interspersed with valleys, the system including a predictive speech decoder for determining predictive parameters of the received signal, and a post filter intended for the production on the output of a signal modified in dependence on the predictive parameters, for increasing the slopes and deepening the valleys.
The importance of the post filtering is to augment the audio quality of speech signals in view of improving the intelligibility and thus better responding to the needs of the users. In the following, the term post filter designates a filter intended to improve the audio quality of the signal.